1. Field of the Invention
The invention relates to a shift control device and a shift control method of an automatic transmission. In particular, the invention relates to a compulsory end timing period for compulsorily ending the shifting action through an oil pressure control or the like if the shift does not end.
2. Description of the Related Art
In a shift control device of an automatic transmission that establishes a plurality of gear steps of different speed change ratios by selectively engaging a plurality of friction engagement devices, it has been proposed that a shift start time point or an inertia start time point is defined as a reference time point, and that if a shift does not end although the elapsed time from the reference time point has exceeded a predetermined time, a post-shift state is attained by compulsorily ending a shifting action, for example, a change in the state of engagement of a friction engagement device, a change in the rotation speed of an input shaft, etc., through an oil pressure control or the like. An example of such a technology is described in Japanese Patent Application Laid-Open Publication No. 2002-89696, in which if a shift does not appropriately end due to an abnormality or the like, the compulsory ending of the shifting action will secure a durability of friction engagement devices, and will assure the end of the shift control.
Since the shift control of an automatic transmission (the pattern of change of the oil pressure control, etc.) is determined through synthetical consideration of responsiveness, shift shock, etc. as well as the durability of friction engagement devices, a shift-required time period in the case where the shift is properly performed varies depending on the vehicle drive state or the like. For example, in an upshift during a power-on state where the accelerator is turned on, the input shaft rotation speed that is apt to rise needs to be compulsorily declined by engaging a corresponding friction engagement device, and therefore the burden exerted on the friction engagement device is large. Hence, the shift control is performed so that the shift ends in a relatively short time. Whereas this shift-required time period is relatively short, the shift-required time period of an upshift during a power-off state where the accelerator is turned off is relatively long. In such an upshift, since a corresponding friction engagement device is engaged after the input shaft rotation speed has naturally declined due to engine friction and the like, the shifting action takes a relatively long time and, besides, the burden on the friction engagement device is small. Thus, the shift-required time period is generally shorter at the time of a power-on shift than at the time of a power-off shift.
It is desirable that the predetermined time period for compulsorily ending a shifting action be set on the basis of the aforementioned shift-required time period. It is also desirable that the predetermined time period for compulsorily ending a shifting action be set shorter at the time of a power-on shift than at the time of a power-off shift. However, in the case where a power-off shift is switched to a power-on shift due to the turning-on of the accelerator halfway through the power-off shift, the corresponding switching of the aforementioned predetermined time period may sometimes result in a shift shock or the like. Specifically, if, in that case, the predetermined time for the power-on shift has already expired at the time point of the switch to the power-on shift, the shifting action is immediately compulsorily ended with possible occurrence of a shift shock or the like, even in the case where the power-off shift was progressing properly, or in the case where although the shift during the power-off state was abnormal the shift control during the power-on state is to be properly performed.